Electric vehicle detection for roadway wireless power transfer

ABSTRACT

A method of detecting a wirelessly powered vehicle on a roadway uses detection of induced voltages or currents in coils provided in the roadway. Once the vehicle has been detected the appropriate coils can be energised to power the vehicle.

FIELD OF THE INVENTION

This invention relates to powering or charging electric vehicles (EVs)wirelessly using inductive power transfer (IPT), and has particularrelevance to detection of an EV on a roadways so that appropriateregions or sections of a roadway can be energised at the appropriatetime to make power available to the vehicle.

BACKGROUND

Inductive Power Transfer (IPT) is a technology which allows power to betransferred wirelessly over a large air gap. This technology has beenproposed for use to charge EVs on roadways and is currently beingstandardised for stationary charging applications for vehicles onroadway surfaces such as in garages and car parks. This highly resonantIPT can also be used in applications requiring dynamic power transferwhere for example a vehicle is able to be powered and/or charged as itmoves over an electrified path, road or highway. This can enable an EVto potentially travel an unlimited distance over these electrifiedroadways without needing to stop for a charge. Combined with recentdevelopments in self driving technology, there is a potential torevolutionise both consumer and private transportation options. However,most IPT research for EV charging is currently focused on stationarycharging and there are still several important issues to solve fordynamic power transfer to be practical.

Dynamic powering is most commonly proposed to be implemented over longsections of road for EVs by ensuring that while driving on the sectionthe vehicle experiences either a continuous or pulsed wave of magneticfield. This is currently achieved by either: a) using an arrangementwhich energises sections of road that are much larger than the length ofa typical vehicle, or; b) using individual modules in the roadway, themodules being similar to, or smaller, in scale to the length of avehicle, and energising these sequentially as a vehicle moves over them.

There are currently several track based solutions which generate acontinuous magnetic field over a section of roadway that is much longerthan the length of the vehicle. The difficulty with this type ofsolution is that the inverters required to drive these tracks have avery high VA rating, meaning that this option is expensive to implement.

While implementing dynamic charging using several individually energisedpads in the road requires significantly more inverters, these inverterscan be rated to a lower VA and these individual systems have thepotential to be more efficient since unused sections of the road are notenergised. One method of implementing the individually energised pads isto use a double coupled system proposed in International PatentPublication WO2011016736. This method utilises a long track operated ata low frequency (10-20 kHz) to distribute power to several independentbut synchronised 85 kHz inverters which sequentially energise pads, asshown in FIG. 1 . Referring to that Figure, a roadway wireless vehiclecharging system is shown diagrammatically. Reference throughout thisdocument to the term “pad” or “pads” refers to magnetic structures whichare used for coupling magnetic fields to each other for the purposes ofpower transfer. In practice, the structures usually include at least onecoil of electrically conductive material, and frequently (although notnecessarily) a magnetically permeable material. These structures may beprovided in modular form, with each module containing one or more coils,as described in international patent publication WO2011016736. Thestructures which are provided as modules located on or in the roadwayare referred to in this document as wireless charging modules, orprimary pads, or ground pads. The magnetic structure provided on theelectric vehicle which is receiving power for the purposes of itsoperation and/or to charge a battery provided in the vehicle is referredto as vehicle magnetic coupling apparatus, or vehicle pad, or secondarypad. It will be understood by the reader that, while this documentrefers to roadway systems which operate to charge the vehicle, thisincludes situations in which power is being supplied to the vehicle butthe vehicle is not actually being charged. Also, while this documentsrefers to roadways, the subject matter disclosed is also applicable toother wireless vehicle systems, such as AGVs for use in industrial ormanufacturing environments for example.

In FIG. 1 the box 2 represents apparatus which is provided as part ofthe infrastructure relating to the roadway. This apparatus can beprovided in or on the roadway, or adjacent to the roadway. The groundpads 4 must be provided in or on the roadway in a location in which avehicle pad 6 passes over them when the vehicle is moving over theroadway. The direction of movement of the typical vehicle pad isillustrated by arrow 8. Therefore, the vehicle pad moves sequentiallyover a series of ground pads 4. Each ground pad 4 is inductively coupledto a track-way 10 which comprises a loop of conductive material that isdriven by an appropriate power supply which may for example include Hbridge 12 that is supplied with power from a utility supply 14.Appropriate power factor control 16 and rectification 18 are provided asrequired.

A considerable challenge to overcome with having individually energisedground pads is to provide a reliable and robust system for detectingwhen to turn on each ground pad. In a highway scenario the vehicle maybe moving at very high speeds of up to 100 km/h (62 mph or 27.7 m/s).Assuming that each ground pad is able to power a vehicle over one meter,this means that each ground pad will only be on for approximately 36 msfor every vehicle. To attempt to establish reliable radio communicationswith each pad within this time is impractical.

Existing solutions for detecting the location of a secondary pad oftenrely on extra hardware such as additional detection coils, such as thesystem disclosed in International Patent Publication WO2014073990.However these methods require additional hardware and apart from cost,ensuring robustness of the in-ground system means ideally minimisingthis.

Another difficulty is that the magnetic topologies for the primary andsecondary pad may be unknown since EV manufacturers may decide to usevarious magnetic topologies in the vehicle which the primary pad needsto be able to tolerate. Thus any viable detection system must besufficiently robust to accommodate situations in which the magnetictopology of the vehicle may be unknown, or when the full system issimply not known.

OBJECT

It is an object of the invention to provide an apparatus, system ormethod which overcomes one or more of the disadvantages of the priorart, or which at least provides a useful alternative.

SUMMARY

In one aspect there is provided a method for supplying power to avehicle on a roadway comprising a plurality of wireless power transfercoils, the method comprising:

detecting a current or voltage induced in a de-energised wireless powertransfer coil, and

energising the wireless power transfer coil dependent on a property ofthe induced current or voltage to make power available to the vehicle.

The method may further comprise the step of opening or closing a switchto make the de-energised coil resonant at a required frequency such thatthe induced current or voltage is resonant.

Detection of the induced current or voltage may further compriseoperating the switch at intervals to allow sampling of the inducedcurrent or voltage. In one example a de-energised tuned coil is switchedto one state to allow the induced current or voltage to resonate, andswitched to another state to allow the induced current or voltage to benon-resonant.

In one example the power transfer coils may be provided in moduleslocated in or on the roadway, and one or more coils may be provided ineach module.

The property of the induced current or voltage may comprise magnitude orchange in magnitude of the induced current or voltage.

The switch may comprise a plurality of switches which may be used toenergise one or more power transfer coils from a power supply. In oneexample the plurality of switches comprise a push-pull converter. Inanother example the plurality of switches comprise an H-Bridge.

In another aspect there is provided a wireless power transfer system forwirelessly powering a vehicle on a roadway comprising a plurality ofwireless power transfer coils, the system comprising:

a switch associated with each coil to selectively energise the coil tomake power available to the vehicle, or de-energise the coil;

a current detector associated with each coil for detecting a currentinduced in the coil when the coil is de-energised;

a controller to control the switch to energise the coil dependent on anoutput of the current detector.

In one example the switch may also be controlled to make thede-energised coil resonant at a required frequency such that the inducedcurrent is resonant.

Detection of the induced current or voltage may further compriseoperating the switch at intervals to allow sampling of the inducedcurrent or voltage. In one example a de-energised tuned coil is switchedto one state to allow the induced current to resonate, and switched toanother state to allow the induced current to be non-resonant.

In one example the power transfer coils may be provided in moduleslocated in or on the roadway, and one or more coils may be provided ineach module.

The property of the induced current or voltage may comprise magnitude orchange in magnitude of the induced current.

The switch may comprise a plurality of switches which may be used toenergise one or more power transfer coils from a power supply. In oneexample the plurality of switches comprise a push-pull converter. Inanother example the plurality of switches comprise an H-Bridge.

In another aspect there is provided a vehicle detection circuit fordetecting a vehicle on a roadway comprising a plurality of wirelesscharging modules, the circuit being operable to detect a change incoupling between a second wireless charging module and a first wirelesscharging module due to the presence of the vehicle.

Preferably the change in coupling comprises a change in coupling betweenthe wireless charging modules due to a wireless power receiver of thevehicle. In one embodiment a permeable magnetic material of the wirelesspower receiver facilitates coupling between the charging modules.

Preferably the change in coupling occurs when the wireless powerreceiver of the vehicle is located between or spans the chargingmodules.

Preferably the change in coupling is detected by detecting an inducedresonant signal. In one embodiment the signal is a resonant current.

Preferably the resonant signal is induced in the second wirelesscharging module from a first wireless charging module via the wirelesspower receiver of the vehicle.

In another aspect there is provided a vehicle detection circuit fordetecting a vehicle having a wireless power receiver on a roadwaycomprising a plurality of wireless charging modules, the circuit beingoperable to detect a resonance induced in a second wireless chargingmodule from a first wireless charging module via the wireless powerreceiver of the vehicle.

In another aspect there is provided vehicle detection apparatus fordetecting a vehicle having a wireless power receiver on a roadwaycomprising a plurality of wireless charging modules, the apparatuscomprising the vehicle detection circuit of the preceding statement.

Preferably the detection circuit detects a resonant current.

In another aspect there is provided a roadway system comprising avehicle detection circuit or vehicle detection apparatus according toone of the preceding statements.

In another aspect there is provided a method of vehicle detectioncomprising the steps of detecting a change in coupling between a secondwireless charging module and a first wireless charging module due to thepresence of a vehicle.

Preferably the method includes allowing the second wireless chargingmodule to resonate. The resonant current or voltage may be detected.

In another aspect there is provided a method of detecting an electricvehicle comprising the steps of:

energising a first wireless power transfer coil to make power availableto the vehicle;

detecting an induced resonance, or a change in an induced resonance, ina second de-energised wireless power transfer coil located adjacent tothe first coil in the direction of travel of the vehicle.

The method may include energising the second coil dependent on aproperty of the induced current or voltage to make power available tothe vehicle.

Preferably the induced resonance is coupled into the second coil fromthe first coil by a magnetic coupling structure of the vehicle.

The method may further comprise the step of opening or closing a switchto make the de-energised coil resonant at a required frequency.

Detection of the induced current or voltage may further compriseoperating the switch at intervals to allow sampling of the inducedcurrent or voltage. In one example a de-energised tuned coil is switchedto one state to allow the induced current or voltage to resonate, andswitched to another state to allow the induced current or voltage to benon-resonant.

In one example the power transfer coils may be provided in moduleslocated in or on the roadway, and one or more coils may be provided ineach module.

The property of the induced resonance may comprise magnitude or changein magnitude of the induced resonant current or resonant voltage.

The switch may comprise a plurality of switches which may be used toenergise one or more power transfer coils from a power supply. In oneexample the plurality of switches comprise a push-pull converter. Inanother example the plurality of switches comprise an H-Bridge.

In another aspect there is provided a vehicle detection circuit fordetecting a vehicle on a roadway comprising a plurality of wirelesscharging coils, the circuit being operable to detect a magnetic couplingbetween a magnetic coupling apparatus associated with the vehicle and atleast one of the wireless charging coils.

Preferably detection of the magnetic coupling comprises detection of acurrent or voltage in at least one charging coil from a magnetic fieldproduced by the magnetic coupling apparatus associated with the vehicle.

In another aspect there is provided a method for detecting a vehicle ona roadway comprising a plurality of wireless charging coils, the methodcomprising detecting a magnetic coupling between a vehicle magneticcoupling apparatus associated with the vehicle and at least one of thewireless charging coils.

Preferably detection of the magnetic coupling comprises detection of acurrent or voltage in the at least one charging coil from a magneticfield produced by the vehicle magnetic coupling apparatus.

In one embodiment the method includes energising the vehicle magneticcoupling apparatus. The vehicle magnetic coupling apparatus may have acurrent provided in a coil of the apparatus which has a magnitude orfrequency or phase which may be detected by the at least one wirelesscharging coils.

In another aspect there is provided a method for detecting a vehicle ona roadway comprising a plurality of wireless charging modules, themethod comprising:

detecting a magnetic coupling between a vehicle magnetic couplingapparatus associated with the vehicle and a first wireless chargingmodule, and

detecting a change in coupling between a second wireless charging moduleand the first wireless charging module due to the presence of thevehicle magnetic coupling apparatus.

Those skilled in the art will appreciate that other aspects of theinvention may become apparent from the following disclosure.

DRAWING DESCRIPTION

One or more embodiments of the invention will be described by way ofexample with reference to the accompanying drawings, in which:

FIG. 1 is a diagrammatic view of a known type of dynamic roadway vehiclewireless charging system.

FIG. 2 is a simplified circuit diagram showing a coupled IPT system.

FIG. 3 a shows measurement of the open circuit voltage in the system ofFIG. 2 .

FIG. 3 b shows measurement of short circuit current in the system ofFIG. 2 .

FIG. 4 shows a circuit diagram of a coupled IPT system with three coils,coil L₁ representing a coil provided on a vehicle, and coils L₂ and L₃representing coils provided in a roadway.

FIG. 5 a is a diagrammatic illustration of a vehicle coil moving overtwo pad structures provided in a roadway

FIG. 5 b is a side elevation of FIG. 5 a.

FIG. 5 c shows the pad structure of FIGS. 5 a and 5 b in plan andelevation.

FIG. 5 d shows a diagrammatic overview including coupling factorsbetween the coils shown in FIG. 5(a) to (c).

FIG. 6 shows plots of coupling against displacement as a vehicle padmoves relative to two stationary pads.

FIG. 7 a shows diagrams of coupling between two ground pads and avehicle pad when viewed in side elevation, FIG. 7 a showing the vehiclepad centred over a first ground pad, and FIG. 7 b showing the vehiclepad located between the two ground pads.

FIG. 8 a shows open circuit voltage induced in a ground pad againstdisplacement of the vehicle pad.

FIG. 8 b shows the plot of three current resonant build up in a tankcircuit associated with the ground pad against displacement of thevehicle pad relative to the ground pad.

FIG. 9 shows an equivalent circuit for the build-up of free resonantcurrent referred to the preceding Figure.

FIG. 10 shows an equivalent circuit which is used to model the effectsof energized coils on free resonant current.

FIG. 11 shows a plot of free resonant current build up which includesthe effects of an energised vehicle pad.

FIG. 12 is a contour plot showing pre-resonant current variation astuned frequency and an equivalent series resistance varies at k=0.01.

FIG. 13 shows a circuit diagram of a series tuned power supply.

FIG. 14 shows plots of coupling and pre-resonant current plotted againstdisplacement with pulsed operation.

FIG. 15 shows a simplified circuit diagram of free resonant currentbuild up with partially parallel compensated series (LCC) tuning.

FIG. 16 shows a bridge circuit driving an LLC compensation circuit(ignoring capacitor C_(S)), or the power supply using LCCL compensationcircuit (if capacitor C_(S) is used).

FIG. 17 shows an equivalent circuit showing three resonant current buildup when the LCL filter is a) open, and b) shorted by the H bridge.

FIG. 18 shows a pushable converter with optional partial seriescompensation and capacitor C_(S).

FIG. 19 shows an equivalent circuit which shows build up of freeresonant current with the push pull converter referred to the precedingFigure.

FIG. 20 shows magnetic fields coupled between ground and vehicle padswhen there is no coupling between L_(G1) and L_(G2) when (a) the vehiclepad is above L_(G1) and (b) when the vehicle pad is between L_(G1) andL_(G2).

FIG. 21 shows an equivalent circuit of the two ground pad and vehiclepad system.

FIG. 22 shows a comparison of free resonant current amplitude withoriginal k₁₂ and k₁₂=0

FIG. 23 shows a diagrammatic system for detecting and supplying powerinductively to a vehicle.

FIGS. 24 and 25 show oscilloscope traces for a simulated system.

DETAILED DESCRIPTION

The description below describes a method and apparatus for detecting thepresence of a vehicle. The vehicle has a vehicle pad which is a magneticstructure including one or more coils for receiving or intercepting amagnetic field created by one or more primary pads provided on or in theroadway on which the vehicle is traveling. Primary pads are alsomagnetic structures having one or more coils. These coils areselectively energised typically in sequence, to create a magnetic fieldwhen the vehicle pad is in proximity to receive the magnetic fieldproduced by the energised coil. Thus primary pads, or at least the coilsof primary pads, are normally in a de-energised state, but need to beenergised at the correct time when the vehicle is in the correctlocation relative to the primary pad, so that power is transferred tothe vehicle. Once the vehicle has passed the primary pad, the primarypad is then de-energised again. In one aspect detection is performed bydetecting changes in coupling between neighbouring primary pads (alsoreferred to herein as ground pads or charging modules) in the roadway.If a roadway vehicle charging system such as that described withreference to FIG. 1 is used, then the detection method can beimplemented without using any additional hardware and without requiringany external sensors, so the system is very robust and inexpensive toimplement.

Those skilled in the art will appreciate that this disclosure is alsoapplicable to systems other than those of the general form described inFIG. 1 . This disclosure is applicable to other systems that may or maynot have a series of primary magnetic structures that are spaced at adistance such that coupling, or a change in coupling between two or morepads can be detected due to the presence of a secondary magneticstructure.

A first embodiment of the invention will now be described. The changesin coupling are detected by detecting or sensing variations in the freeresonant current developed in neighbouring primary pads created due tothe small variations in coupling between neighbouring pads. Theincreasing coupling between the primary pad from the approachingsecondary pad may also be used as the detection means. These variationsin the free resonant currents may be detected by current transformerswhich are already present to help monitor and control the primary padcurrent. Measurement may be performed of current in a coil, or in aninverter associated with a coil. By observing the magnitude of the freeresonant currents, the upcoming secondary pad can be accurately detectedregardless of what magnetic topology is used for both the primary andsecondary pads, and ideally regardless of various types of invertertopologies. The proposed method is also able to detect the secondary padeven if it is un-energised and can be used with a variety of primaryinverter topologies, for example H bridge driving various compensationcircuits such as a series LC, LCC, LCL as well as the typically currentsourced converters such as push-pull converters. The only condition thatis required for this detection technique to work is that the pads arespaced in such a way that it is possible for a voltage or current to beinduced in a neighbouring primary pad when the vehicle transitions fromone pad to the next.

Some fundamentals of IPT will now be described to assist withunderstanding the detection method. A simple IPT circuit is shown inFIG. 2 where the groundpad L₁ which is tuned by the capacitor C₁ ismagnetically coupled by the coupling factor k to the vehicle pad L_(V)which is tuned by C₂. L_(G) and L_(V) are tuned to the desired frequencyof operation (85 kHz is the example assumed for much of the followingdisclosure) as shown:

$\begin{matrix}{\omega = {\frac{1}{\sqrt{L_{G}C_{1}}} = \frac{1}{\sqrt{L_{V}C_{2}}}}} & (1)\end{matrix}$

The mutual inductance (MGV) between LG and LV is given by:

M_(GV)=k√{square root over (L_(G)L_(V))}  (2)

where k is the coupling between the two inductors.

When LG is excited by a sinusoidal current, the voltage induced in LV(also referred to as the open circuit voltage −V_(OC)) shown in FIG.2(a) and its short circuit current (I_(SC)) shown in FIG. 2(b) is givenby:

$\begin{matrix}{V_{OC} = {j{\omega M}_{GV}I_{1}}} & (3)\end{matrix}$ $\begin{matrix}{I_{SC} = {\frac{V_{OC}}{j{\omega L}_{V}} = {\frac{M_{GV}I_{1}}{L_{V}}.}}} & (4)\end{matrix}$

The currents flowing in a system having three coils with only one coilenergised as shown in FIG. 4 can be obtained by solving the linearsystem. Furthermore, a generalised system can be written for an n-coilsystem. These equations will be used to enable steady state analysis ofmany of the systems presented in this specification and allow manyfeatures to be taken into account such as the losses in the componentswhich are represented as equivalent series resistances and imperfecttuning conditions.

To provide an example to describe an embodiment of the method, a simplesimulation of a dynamic charging system consisting of two ground padsand a vehicle pad has been set up in the 3D magnetic FEM program JMAG asshown in FIG. 5(a). A similar set up is used to experimentally validatethe simulation as described further below. In this embodiment, thespacing between the ground pads was chosen so that the minimum I_(SC) ofthe vehicle pad when it transitions between the two ground pads is 35 Awhen both ground pads are energised. This ensures that the vehicle willalways be able to draw 10 kW assuming it uses a 300 V bus. For the sakeof this example the primary or ground pads are assumed to be driven by aconstant current source of 100 A which can be practically achieved withan LCL converter. The ground pads use the DD magnetic topology describedin International patent publication WO2010090539 in order to produce ahorizontal field while the vehicle pads could utilise the DD, DDQ or thebipolar magnetic topology such as that described in International PatentPublications WO2011016731 and WO2012018269. In this case the bi-polarmagnetic topology was used. The person skilled in the art willappreciate that the magnetic topology is not critical, since theproposed detection scheme will work with all magnetic topologiessuitable for power transfer including circular pads and ferrite-lessground pads, provided pads are located in sufficient proximity to eachother and/or the vehicle pad contains a suitable permeable material,such as ferrite for example.

As the vehicle pad moves over the ground pads as shown in FIG. 5 , thereis a variation in coupling factors between all three pads in the systemas shown in FIG. 6 . The coupling between the ground pads and thevehicle pads (K_(G1V) and K_(G2V)) increases when the vehicle pad isbetween the two ground pads. This is because when the vehicle pad iscentred on the ground pad there is very little leakage flux generated bythe energised ground pad that can be coupled into the neighbouringground pad as indicated in FIG. 7(a) since the flux or field is mostlyattracted to the vehicle pad. When the vehicle pad is between the twoground pads the leakage magnetic flux coupling from the energised groundpad into its neighbouring ground pad increases since the ferrite in thevehicle pad gives the flux an alternative magnetic path or pathway toflow through as indicated in FIG. 7(b). This enhancement of the leakagefield occurs since the spacing of the ground pads is designed to ensurethat the vehicle can draw power almost continuously from the roadway,ideally 10 kW, in all positions. If the ground pads were spaced furtherapart the variation in coupling between neighbouring ground pads mightbe too low to work as a reasonable means of detection.

The voltage induced in the neighbouring pad is shown in FIG. 8(a). Whilethis voltage of 25 V to 75 V is suitably large for detection means, itis very small compared to the several kilovolts that appears across thepad when it is energised, so any detection circuit that attempts todetect this change in induced voltage has to also withstand severalkilovolts, making this approach less desirable. However, if theswitching network is operated in such a way when it is not trying todeliver power so that it configures the LC circuit to resonate in thepresence of a coupled voltage at a frequency at or near its tuned point,a large current can build up in the LC tank, allowing for easydetection. The practical implementation of how to allow the resonantcurrents to build up depends on the power supply topology and will bediscussed further below. Most converter topologies can be simplified toa single switch as shown in FIG. 9 when not transferring power, and hereV_(OC) is the voltage induced by L_(G1), C_(G2) is the tuning capacitorto tune L_(G2) and r_(ESR) is the AC resistance of all the components inthe loop.

The steady state current induced in the pad as the coupling varies isshown in FIG. 8(b). It can be seen that the nominal current when thevehicle is centered above the pads is approximately 25 A and itincreases to approximately 70 A as the vehicle moves between the twoground pads. As noted earlier, the current transformer present in theloop is usually used to monitor the current in the ground pads, thischange in free resonant current from 25 A to 70 A (or similar based onapplication) can be used as a means of detection.

The analysis above has assumed that the vehicle pad has no currentflowing through it and that no power is being drawn. The proposeddetection scheme also works when the vehicle pad is energised. Tovalidate this, the circuit to model this system is shown in FIG. 10 andthe change in the free resonant current flowing through L_(G2) is shownin FIG. 11 with the vehicle pad energised. In this example the loadedquality is adjusted to draw 10 kW while limiting the maximum loaded Qfactor of the secondary to 10.

It can be seen in FIG. 11 that the free resonant current in theneighbouring ground pad L_(G2) still produces a clear current signalwhich allows detection of when the vehicle pad is centred between thetwo neighbouring (adjacent) ground pads.

As noted earlier, the functionality of the presented circuit largelydepends on the free resonant LC tank being tuned to approximately thesame frequency as the energised primary as well as having low losses.This can be observed in FIG. 12 which shows contour curves of theresonant current that builds up as the frequency and losses changes atk=0:01. It can be seen that as the system is detuned, the resonantcurrent decreases, making it more difficult to detect the change in freeresonant currents. The resonant current also decreases as the ESR of theLC tank increases however if the ESR is very low then the currentincreases towards infinity. However, any free resonant current detectedin the unenergised neighbouring ground pad will still approximatelytriple given the coupling factor approximately triples as the vehicletravels off the center of the energised ground pad. This means that thisrelative change in current magnitude can be detected as long as thecurrent reading is of a high enough resolution.

As mentioned previously, the free resonant current detection scheme canbe implemented on multiple electronic topologies, however each topologyhas a slightly different way of implementing it. The implementation ofthe detection scheme with an H-bridge series tuned, H-bridge LCL tunedand a push-pull inverter is discussed below.

A series tuned LC power supply is shown in FIG. 13 . Typically, when thepower supply is shut down all four switches are left open. To allow thecircuit to freely resonate the bottom switches need to be closed whileleaving the top switches open. When this happens the circuit canessentially be redrawn as FIG. 9 . This circuit has already beenanalysed above, however one of the disadvantages with the circuit isthat continuously having some current flowing through the coils canintroduce additional loss which is not necessary. To get around thisissue the converter can operate the bottom switches in such a manner sothat it only allows pulses of resonant current to build up within the LCtank. By operating the circuit in the short circuited state 10% of thetime and open circuited the rest of the time, the losses in the freeresonant coil can be significantly reduced.

This pulsed operation of the H-bridge is shown in FIG. 14 where the LCtank is only short circuited for 10% of the time at a low frequency of 1kHz. When the switches are opened the energy in the LC tank isdischarged into the input DC bus capacitor through the body diodes ofthe MOSFET. However, since there is very little energy in the LC tankcompared to the DC bus capacitor, the energy decays within one cycle.The pulsed operation of the H-bridge will not be as effective in aseries LCC circuit if a partially parallel tuned circuit is used asshown in FIG. 15 . When the bottom switches of the H-bridge are openedup, resonant currents can still circulate within the loop created byL_(G2) and Cparallel. The amplitude of these circulating currents willhowever be lower as expected from FIG. 12 since the circuit will not beperfectly tuned. This will mean that when the H-bridge is opened thecircuit will still resonate but at a lower current instead of decayingto 0 A with the series LC circuit.

The typical configuration of an LCL and LCCL tuned power supply is shownin FIG. 16 . The LCL filter is tuned to the operating frequency bychoosing values of C₁ and Lin such that C₁ and L₁ as well as C1 and Linare tuned to w as shown in (6). Often this is achieved by making Linequal to L_(G2).

The tuning equation for the LCL converter (6) can be modified for theLCCL converter by making Lin equal to the combined impedance of L_(G2)and C_(S) when operating at w was shown:

Due to the way the circuit is tuned, the large resonant current can beobserved in two places—both within the pad when the H-bridge presents anopen circuit and within L_(in) when the H-bridge presents a shortcircuit to the filter as shown in FIGS. 17(a) and (b) respectively.

When the H-bridge presents an open circuit (by having all the switchesopen) there is no current that flows through L_(in) and a large resonantcurrent that flows through L_(G1) which can be detected by the currenttransformer which monitors the pad current. However if the LCL filter isshorted by turning on both of the bottom switches of the H-bridge, nocurrent will flow through L_(G2) and the free resonant current willbuild up in L_(in). The resonant current in L_(in) can be detected bythe current transformer that monitors the bridge current.

Unlike the series tuned supply in above, the resonant current cannot beturned off so the inductor which should be used for resonance should bechosen by using the lowest loss inductor. However, as a result of thisthe circuit is not impacted by any partial tuning of either the L_(in)or L_(G1).

The push-pull inverter such as the one shown in FIG. 18 can also utilisethe free resonant current in order to detect upcoming pads. Typically C₁is selected so that it tunes L_(G2) to w similar to how an LCL circuitis tuned in (6). Sometimes an optional partial series compensationcapacitor C_(S) is used to increase the current through L_(G2) in whichcase the equivalent inductance C₁ should be tuned to the equivalentinductance of L_(G2) and C_(S) similar to how the LCCL circuit is tunedin (7). The equivalent circuit of the converter when it is operating infree resonant mode is shown in FIG. 19 . When the switch is open(corresponding to both push-pull converter switches being open) theparallel tuned resonant tank is allowed to resonate freely. However whenthe resonant switch is closed (corresponding to both push-pull converterswitches being closed) the pad is effectively shorted so the resonantcurrents will not build up.

Instead the short circuit current will flow through the pad if C_(S)isn't used. If the partial series compensation capacitor C_(S) is usedthen some resonant currents will still circulate within L_(G2) and C_(S)however the amplitude of these circulating currents will be low sinceL_(G2) and C_(S) do not resonate at w.

The detection scheme with the push-pull converter is similar to theseries tuned circuit because the pulsing approach can also be used withthe push-pull converter. However if the circuit is partially seriestuned then some resonant current will still build up as is the case withthe series tuned power supply.

In another embodiment, the current flowing within the approachingvehicle magnetic coupling structure (i.e. the coil) can be detected bythe approaching vehicle pad as shown in FIGS. 20(a) and 20(b). In FIG.20(a) the vehicle pad is above L_(G1) so there is effectively nomagnetic fields generated by L_(V) that couple into L_(G2) since L_(V)is far away from L_(G2). However as the vehicle pad approaches theL_(G2), the magnetic field generated by L_(V) starts to also couple intoL_(G2) as shown in FIG. 20(b). Electrically this system can be drawn asFIG. 21 however since there is no coupling between L_(G1) and L_(G2),i.e. k₁₂ is zero. In other words, the detection system still works ifk₁₂ is zero.

A simulation comparing the free resonant current induced when theeffects of including the original k₁₂ is compared to setting k₁₂=0 isshown in FIG. 22 . It can be seen that initially when the LV is aboveL_(G1) (displacement of 0 to 0.3 m) there is approximately 20 A of freeresonant current flowing with the original k₁₂ however there is lesscurrent with k₁₂=0—especially at zero displacement since there is almostno magnetic field from L_(V) coupling into L_(G2). As the vehicle movesinto the gap between the two ground pads, the coupling between L_(G2)and L_(V) increases, causing a significant increase in IL_(G2), eventhough there is no coupling between the two ground pads. After adisplacement of 0.5 the IL_(G2) starts to drop again however by now thepower supply driving L_(G2) should have detected the vehicle and turnedon.

In FIGS. 20(a) and 20(b) and FIG. 22 , the ground pad L_(G1) istransferring power to L_(V) so there is a current flowing through thevehicle pad. This current in the vehicle pad generates the magneticfield that induces the free resonant currents in L_(G2) which are usedfor the detection. The detection scheme does not have to be limited tojust the vehicle pad though—if the vehicle used intermediate couplers toimprove power transfer between the ground pad and the vehicle then thecurrent generated by the intermediate couplers will induce free resonantcurrents in the upcoming ground pad which can also be used as a means ofdetection. In fact, any energised coil(s) which has an increasingcoupling to the next ground pad as the vehicle approaches that groundpad can be used as a means of detection. This can be realisedpotentially having a detection coil at the front of the vehicle which issimply used to indicate to the upcoming vehicle pads that the vehicle isapproaching. The frequency and amplitude of the current in thisdetection coil can also be adjusted to indicate to the ground padinformation about the desired primary current. Alternatively smalldetection coils which are energised can be laid under the ground, justbefore the next to the ground pad. The upcoming vehicle would increasethe coupling between the detection coil and the next ground pad whichcan also be used as a means of detection.

The detection scheme shown in FIGS. 20(a) and 20(b) and FIG. 22currently works because the two ground pads are still close enough sothat the vehicle pad is still coupled to both ground pads. However ifthe highway system was designed so that there are large gaps between theground pads (say 0.5 m or even higher) then the vehicle pad can simplybe energised to indicate to the next ground pad that it is approaching.An easy way of implementing this type of detection scheme is to use anactive rectifier such as the ones used in bidirectional wirelesscharging systems. If the active rectifier keeps switching even after ithas stopped receiving power from the previous ground pad then thevehicle pad will still have currents flowing within it. Once the vehiclegets close enough to the next ground pad, the current flowing in thevehicle pad will induce free resonant currents in the next ground padwhich will indicate to the ground pad to turn on.

If a traditional passive rectifier is used, the currents in the vehiclepad would stop as soon as the ground pad is no longer transferring powerto the vehicle pad so an active switching rectifier or an additionalinverter on the vehicle side is required to energise the vehicle pad, orthe ground pads need to be placed closer together in the road. In oneembodiment, the vehicle pad can be energised using a minimal amount ofpower, so that a field which is sufficient for detection is produced, toensure that the detection process is efficient.

An energised vehicle pad can also be used to indicate to the firstground pad to turn on in a new series of ground pads, so the vehicle padcan start the chain of all the other ground pads turning on in sequenceas the vehicle passes over them. The operation of the chain of padsafter the first pad has been turned on may be the same as thatembodiment described above with reference to FIGS. 1-19 . In someembodiments, both the embodiment of FIGS. 1-19 and the embodiment ofFIGS. 20-22 may be used.

The free resonant current induced by any energised coil (regardless ofif it is the previous vehicle pad coupling via k₁₂ or a vehicle with anactive rectifier deliberately energising the vehicle pad) can be used toextract the following useful information about the source of theexcitation current:

-   -   Frequency of the excitation current    -   Phase of the excitation current    -   Approximating the amplitude of the excitation current under        controlled circumstances

The frequency of the free resonant current will be exactly the same asthe frequency of the excitation current. If the previous ground pad isrunning at say 84.2 kHz then the current that flows through the vehiclepad will also be 84.2 kHz so the free resonant current induced in theupcoming ground pad will be 84.2 kHz. This can be detected fairly easilyand the next power supply can be energised at exactly 84.2 kHz.

Alternatively, in future if private vehicles use 85 kHz as theirfrequency to transfer power and larger commercial vehicles run at adifferent frequency (say 50 kHz) then this can be detected easily. Theprivate vehicles would energise their vehicle pads at 85 kHz to indicateto the ground pads that it wants to run at 85 kHz and the commercialvehicles would energise their vehicle pads at 50 kHz to indicate that itwants to run at 50 kHz. The practical implementation of this wouldrequire the tuned circuit in the upcoming ground pads to be tuned to asimilar frequency. For example, consider a situation where 50 kHzexcitation current is used to indicate to the upcoming ground pad whichhas its filter set to 85 kHz then the amplitude of the free resonantcurrent would be very low. It will probably still be possible to detectthat it is a 50 kHz signal and switch the tuning to ‘50 kHz Mode’ whichwould then give larger and more accurate readings.

The phase of the free resonant current will have some relationship tothe phase of the excitation current. For example—if the phase of theexcitation current changes by 90° for whatever reason, this will bereflected by the same 90° phase change in the free resonant currents.This means that after the upcoming power supply runs through acalibration phase where it energises its coils to figure out how it istuned, the upcoming power supply will be able to energise its ground padat an exact phase angle relative to the excitation current. If thesystem is designed so that the coupling between the ground pads is usedas a means of detection then the upcoming pads can detect the phase ofthe excitation current and turn on to match that exact angle. Withexisting systems this is beneficial because if the ground pads areoperated out of phase then the ground pads may start transferring powerbetween themselves. The phase detection is also useful if the system isdesigned so that the vehicle pad indicates to the ground pad when toturn on by using an active rectifier to generate currents in the vehiclepad. The phase of the ground pad can be set to be 90° leading or laggingrelative to the phase of the primary current to transfer power inwhichever direction desired.

In FIG. 22 the initial current of the blue trace (when there is couplingbetween the ground pads) is approximately 20 A at a displacement of 0 m(when L_(G1) is energised at 100 A). If the graph was to be extendedback to −1 m then the current reading will still be approximately 20 Aassuming L_(G1) was still energised at 100 A since k₁₂ would not havechanged. However if L_(G1) was energised at only 50 A then the initialfree resonant current reading in L_(G2) would be approximately 10 A.This means that if L_(G1) is energised at different current levelsdepending on what the vehicle is then L_(G2) can detect what currentL_(G1) is operating at and match that current. However, this requiresadditional information—the power supply for L_(G2) needs to know thatwhen it sees an initial free resonant current of 20 A it corresponds toL_(G1) being energised at 100 A. This information can be provided bygiving the power supply the nominal value of k₁₂ and have it figure outthe rest based on how it is tuned using a self-calibration routine orsimply telling it that a 20 A reading corresponds to 100 A in the code.

In the simulations we were able to detect a pad turning on from 2-3 padsaway because the free resonant current in one pad induces anothersmaller free resonant current in the next pad.

One of the common questions with bidirectional charging systems, is howthe primary and secondary synchronise themselves since bidirectionalsystems rely on having an accurate phase difference between the primaryand secondary inverters. The proposed detection scheme can helpsynchronise the primary and secondary inverters by following thefollowing steps:

1. The vehicle inverter starts to energise the vehicle pad. This createsfree resonant currents within the ground pad if the ground pad issufficiently coupled.

2. Detect the frequency of the free resonant current—this is thefrequency that the ground inverter needs to operate at.

3. Measure the phase of the free resonant current and do a look up withexisting tuning data from the self-calibration routines to determine thephase of the current in the vehicle pad.

4. Drive the inverter at the detected frequency and at ±90° to the phaseof the current in the vehicle pad.

The detection scheme would be impacted by slight changes in inductancedue to the pads being aligned/misaligned, but this may also be accountedfor in the design of the system implementation.

Other information can also be passed along via the free resonant method.For example—the power supplies can pulse their pads on/off to ‘transmit’a binary signal or may be ‘transmit’ a modulated signal too when thereis no vehicle pad present.

In FIG. 22 the peak free resonant current occurs at a displacement of0.5 m. However inverter does not need to wait till the displacement of0.5 m to turn on the next ground pad. For example—the power supply candetect that the free resonant current has increased to approximately35-40 A from an initial current of approximately 20 A. This is a goodenough indicator to turn on the power supply. The power supply does notneed to wait till the current is 60 A to turn on.

In FIG. 23 an illustration is shown of implementation of the system.Converter 21 includes switches as described above that are operable bycontroller 22 to drive L_(G2) so that it may be energised orde-energised, and to allow the pad to be provided in an appropriatestate to receive or sample induced free resonant currents. The freeresonant current induced in is detected by a sensor such as currenttransformer 20, and information on the magnitude or change in magnitudeof the free resonant current is passed to controller 22. When a certainproperty of the current is detected (for example a threshold magnitude)then the controller can instruct the converter 21 to energise the pad sothat power is made available wirelessly to the vehicle whose movementcaused the induced current in L_(G2).

To experimentally test the system a PLECs simulation was created withfour ground pads energised by LCL power supplies. The results are shownin FIG. 24 where the four ground pad currents and the output DC currentare plotted. The dotted red lines indicates the times when the vehiclepad is centred on top of each ground pad. It can be seen that as thevehicle pad moves from L_(G1) to L_(G4) each power supply turns on andoff to energise each ground pad at 100 A. A switching secondaryregulator was used to regulate the output current to approximately 33 Awhich was fed into a 300 V load to deliver 10 kW continuously. Thedetection of the vehicle pad was implemented by the power supplymonitoring the free resonant current in the ground pads. The vehicle padwas turned on when the free resonant current reached 50 A. This ensuresthat the next ground pad is energised before the vehicle pad is in thegap between the ground pads. The power supply is turned off when thebridge current gets low, indicating that the power supply is no longerdelivering power to the vehicle. The nominal component values used forthe simulation are listed in Table II.

Table II

Nominal Component Values Used in the Dynamic Simulation

TABLE II NOMINAL COMPONENT VALUES USED IN THE DYNAMIC SIMULATION L_(CHN)9 μH L_(G1) 81.9 μH C_(G1) 394 nF C_(GiS) 47 nF L_(V(A/B)) 11.16 μHC_(V(A/B)) 313 nF L_(DCout) 200 μH C_(DCout) 660 μF Q_(L) 625 Q_(C) 1000V_(inDC) 600 V V_(outDC) 300 V

Oscilloscope traces of an experimental the vehicle detection are shownin FIG. 25 . The vehicle pad initially started at a displacement of−0.322 m which was the furthest displacement where the vehicle was stillable to draw 3.3 kW. The vehicle was accelerated so that it wastravelling at approximately 6 km/h at vzero displacement where thevehicle pad is centred over L_(G1). It can be seen that as the vehiclepad continues to move towards L_(G2) the free resonant current (I_(G2))starts to increase. The power supply detects this increase in currentand automatically starts up the power supply for L_(G2) when the freeresonant current has reached 18 A. Both power supplies deliver power tothe vehicle when the vehicle is transitioning from L_(G1) to L_(G2).After the vehicle pad crosses the center of L_(G2) the output currentstarts to drop until it eventually reaches zero. If another ground padwas present then the next ground pad would need to detect the upcomingvehicle and turn on to ensure that the vehicle keeps receiving power.The power supplies shut down after they detect a sufficiently low bridgecurrent which indicates that it is no longer supplying power. Throughoutthis experiment there was no wireless communications between the vehicleand ground pads and no extra detection hardware was used—all thedetection was done by monitoring the free resonant currents. The onlyrequirement was that the power supply for L_(G1) had to be turned on tostart with so that the resonant currents could be induced within L_(G2).For a highway charging application, vehicle sensor coils such as thoseused to detect vehicles at traffic lights could be used to indicate tothe first ground pad to turn on. After the first ground pad turns on, itwill create a chain reaction of all the other ground pads turning on oneby one as the vehicle moves over all of them.

From the foregoing it will be seen that reliable and robust detectionelectric vehicle detection systems and methods are provided.

1. A method for supplying power to a vehicle on a path or roadwaycomprising a plurality of wireless power transfer coils, the methodcomprising: detecting a current or voltage induced in a de-energisedwireless power transfer coil; and energising the wireless power transfercoil dependent on a property of the induced current or voltage to makepower available to the vehicle.
 2. The method as claimed in claim 1wherein the induced current or voltage is resonant.
 3. The method asclaimed in claim 1 wherein the current or voltage is induced in thede-energised coil from energisation of a neighbouring coil.
 4. Themethod as claimed in claim 1 wherein the current or voltage is inducedin the de-energised wireless power transfer coil by the vehicleproviding a magnetic path from a neighbouring energised wireless powertransfer coil.
 5. The method as claimed in claim 1 wherein the currentor voltage is induced in the de-energised wireless power transfer coilby a magnetic field produced by the vehicle.
 6. The method as claimed inany claim 1 comprising a step of opening or closing a switch to make thede-energised wireless power transfer coil resonant at a requiredfrequency such that the induced current or voltage is resonant.
 7. Themethod as claimed in claim 6 comprising operating the switch atintervals to allow sampling of the induced current or voltage.
 8. Themethod as claimed in claim 1 wherein the property of the induced currentor voltage may comprise magnitude or change in magnitude of the inducedcurrent or voltage.
 9. A wireless power transfer apparatus for providinga magnetic field to wirelessly power a vehicle on a path or roadwaycomprising a plurality of wireless power transfer coils, the apparatuscomprising: a switch associated with each coil to selectively energisethe coil to make power available to the vehicle, or de-energise thecoil; a current detector associated with each coil for detecting acurrent induced in the coil when the coil is de-energised; and acontroller to control the switch to energise the coil dependent on anoutput of the current detector.
 10. The wireless power transferapparatus as claimed in claim 9 wherein the controller is operable tomake the de-energised coil resonant at a required frequency such thatthe induced current is resonant.
 11. The wireless power transferapparatus as claimed in claim 10 wherein detection of the inducedcurrent further comprises sampling of the induced current.
 12. Thewireless power transfer apparatus as claimed in claim 11 wherein thecontroller is operable to provide a de-energised tuned coil in a firststate to allow the induced current to resonate, and a second state toallow the induced current to be non-resonant.
 13. The wireless powertransfer apparatus as claimed in claim 9 wherein the controllerenergises the coil dependent on the magnitude of the induced current.14. The vehicle detection apparatus as claimed in claim 22 wherein thechange in coupling is detected by detecting an induced signal.
 15. Thevehicle detection apparatus as claimed in claim 14 wherein the inducedsignal is a resonant current or voltage.
 16. The vehicle detectionapparatus as claimed in claim 15 wherein the detection circuit isoperable to make a de-energised module resonant at a required frequencysuch that the induced current or voltage is resonant.
 17. The vehicledetection apparatus as claimed in claim 15 wherein the detection circuitsamples the induced current or voltage.
 18. The vehicle detectionapparatus as claimed in claim 15 wherein the detection circuit isoperable to provide a de-energised module in a first state to allow theinduced current or voltage to resonate, and a second state to allow theinduced current or voltage to be non-resonant.
 19. The vehicle detectionapparatus as claimed in claim 15 wherein the detection circuit isoperable to energise the module dependent on the magnitude of theinduced current or voltage. 20-21. (canceled)
 22. The method of claim 4,further comprising: detecting a current or voltage induced in anenergized wireless power transfer coil, and deenergizing the wirelesspower transfer coil dependent on a property of the induced magneticfiled in the energized wireless power transfer coil.